133.4

GAIN- AND G/T-CONSIDERATIONS ON SPACEBORNE ACTIVE RECEIVE ANTENNAS

Ulrich R. Kraft, Detlev U. E. Blaschke Dornier Satellite Systems, Dept. RST2,

P.O. BOX 801169, D-81663 Munich, Germany

INTRODUCTION

Due to the increasing need for wide-area communication systems providing capacity on demand to a dynamically changing distribution of customers, a flexible adaption of the provided coverages to the actual needs of the system will be a requirement for the antennas of future, advanced communication satellites. In this area, multi-feed reflector antennas (MRAs) with suitable beam-forming networks (BFNs) are the prefered solution for satellites in the geostationary orbit (GEO), since the GEO requires narrow beams with a limited scan range. For satellites in a low earth orbit (LEO) direct radiating phased arrays can be attractive, since wider beams with larger scan angles are required in this case. Though these two antena types are significantly different in terms of radiator numbers, they basically require the same BFN functions, which comprise the control of amplitude and phase weightings of the individual radiators as well as a combination or distribution of signals between these radiators and the output or input port of the antenna. Whereas a limited reconfiguration capability is already available through MRAs with switched, passive BFNs (e.g. INTELSAT), an advanced coverage control will require the implementation of MRAs or phased arrays with active BFNs providing fully flexible amplitude and phase weightings and a minimization of the intemal antenna losses.

Apart from the technological difficulty to implement an active antenna on-board of a communication satellite, a non-trivial, basic problem for the handling of such antennas within the system analysis and design is the lack of a suitable antenna description on system level, which is based on measurable antenna parameters and which permits an easy adaption of the already established system engineering tools and procedures to this special case. Currently, even such basic terms as the gain of an active antenna are not finally defined [ 11 and only few publications on the subject exist, which are mainly focussed on phased arrays (e.g. [2]).

The goal of our contribution is to provide a more general description for the most system-relevant parameters gain, noise temperature and G/T of an active receive antenna, which covers both MRAs and phased arrays and which is based on measureable antenna parameters. Apart from a use for system analysis and design, this description can be applied also to obtain useful specifications for such antennas and to provide some insight into their basic behavior in comparison to conventional passive types.

0-7803-4178-3/97/$10 00 0 1997 IEEE 2468

GENERAL GAIN AND G/T-DESCRIPTION The general configuration of an active, spaceborne receive antenna is shown in Fig. la. It consists of a N-element radiator system witWwithout reflector described by the obtained directivity D and the internal losses LA and an active BFN characterized by the net-gain GBFN and the noise temperature TBFN. At the antenna-repeater-interface (AN), the antenna is connected with the corresponding repeater, which is described by the noise temperature TRP.

~ ~~

Equivalent Block-Diagram (Top Level) '0, I' ARI

T.h System Network (BM)

I

- ARI

T.h System Network (BM)

Equivalent Block-Diagram (-0- to ARI)

'0. '1' '2'

Uin.1

TA -c

UinN + TA

I I - Fig. l a General configuration Fig. l b 'Internal' block-diagram

The total antenna gain Gtot available at the ARI (antenna output) is given by

Gtot[dBi] = D[dBi] - L A [ ~ B ] + GBFN[~B] (1) which is an extension of the classical formulation by a BFN-term. It can be measured using standard procedures for passive antennas and describes the total signal amplification of the antenna compared to an ideal isotropic radiator, which is the conventional definition of antenna gain. Assuming LA as ohmic losses (worst case), the G/T of the reception chain is given by

G/T[dB/K] = Gt~t[dBi] - 10 log ((TA +Tact) (GBFN / LA) + TRP)

G/T[dB/K] = D[dBi] - 10 log (TA + Tact + TRP (LA/ GBFN))

(2a)

(2b) or

with (2c)

wherein TA, To and Tact denote the classical antenna noise temperature given by the totally received noise [3], the physical temperature of the antenna and the internal effective noise temperature of the active antenna, respectively.

AS shown in Fig. 1 b, the BFN comprises N identical active chains .which perform the amplitude/phase settings (l/Ln, +n) and which are described by a common maximum gain Go, the individual gains Gn=Go/Ln and the setting dependent noise temperatures Tn=T(Ln). All signals are added within a combiner with an overall loss Lc per chain and N noise sources Tcn, which describe the internally generated noise per combiner input.

The gain GBFN of this BFN can be derived from a comparison of the achieved output voltage Uout with the corresponding figure U'out provided by an ideal, lossless BFN (GEFN=~), which is given by

Tact = (LA-1) To + LA TBFN

2469

N

U'out = (l/KA) n=4 C Um,n exp(j+n)/Cn

nan C ( 1 ~ ' " ) = 1 (conservation of energy)

( 3 4

(3b) N with

Using the definition Uout = U'out &, the normalization Ln = L'n/Gw and eqs. 3a and 3b, GBFN can be written as

GBFN = Go Gw / Lc or GBFN[~B] = Go [dB] - Lc[dB] + Gw[dB] (4a) N

with GW =05 (Inn) (4b)

Herein Go - Lc describes the maximum gain per receive chain ('1' to A N ) and Gw characterizes the impact of an applied amplitude weighting on the overall BFN amplification (taper gain). The maximum range of Gw is given by the extremes +10 log N (uniform weighting, 1/Li N = 1) and 0 dB (one chain active, 1/Li = 1, 1/L2 N = 0), which occurs in case of MRAs, where N is moderate but individual radiators are used to form the desired beam. In case of phased arrays, N is much larger but only moderate tapers are applied giving a larger possible Gw-range but a similar effective Gw-variation.

Using Fig. 1, the antenna output noise temperature Tout can be written as the sum of noise contributions from the individual chains Tout,n available at the ARI and as a fimction of the BFN LA and TA ,,

Tout = (GBFNLA) (TA + (LA-I)To + LATBFN) = n C 21 Tout,n ( 5 )

From this expression, TBFN can be derived to /v

n 14 TBFN = (l/Gw) C T(Ln)/Ln + (NT~GoGw) ( 6 4

Tc = ( I N ) Tc," (average combiner noise) (6b) with N

n= 4

which can be simplified to

TBFN = T' + (N/Gw - 1) T" + (NTc/GoGw) (7)

for most practical cases, where T(Ln) can be written as T'+ (Ln-1)T". For these cases, TBFN varies between Tmin = T' + Tc/Go (uniform weighting) and Tmaw = T'+ (N-1) T" +NTc/Go (one chain active).

EXAMPLE : SPOTBEAM MRA WITH ZOOMING CAPABILITY In order to provide an illustrative application example, an active M M with a 81h offset-reflector (F/D = 0.76), a hexagonal 7 horn cluster and a BFN comprising LNAs with GLNA = 20 dB and TLNA = 300 K, resistive amplitude weights Ln and phase shifters with a loss of L$ = 1.0 dB has been analyzed (Fig. 2a). The losses of the radiator system (LA) and the combiner (Lc) have been assumed to 1.0 dB and 9.5 dB, whereby the internally generated combiner noise has been neglected due to the small number of chains.

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1 I Configuration hampie

L. "pi 4 U

I

Fig. 2a MRA configuration Fig. 2b Antenna pattern example

If a ring-taper (central horn : Li = I , ring horns : L2 ... 7 = LR) is applied, the beamwidth of this antenna can be adjusted by changing LR from 2.7 dB to 17.5 dB (Fig. 2b), which changes D (37.4 dBi to 44.3 dBi), Gw (6.2 dB to 0.5 dB) and GBFN (15.7 dB to 10.0 de). The effect on the total gain Gtot

available at the ARI is shown in Fig. 3a, whereas Fig. 3b shows the G/T of the entire reception chain comprising the antenna with TA = 250 K and To =

300 K and a repeater with an effective noise temperature of Tw = 1000 K.

c - d . V

60.00

50,OO

40.00

30.W

20.00

lo.w 2 9 5,M) 7,50 10.W 12.50 15,OO 17,W

Ringtaper (in dE) Ringtaper (in dE)

Fig. 3a Active antenna total gain Fig. 3b G/T of reception chain

It is interesting to note, that the total antenna gain is almost independent from the actual beamwidth (Gtot = 52.1 - 53.3 dBi) since D and GUM change within a similar range but in opposite directions. This can permit a repeater simplification and is different from the phased array case, where D and GBFN change in the same way. The achieved G/T-figure changes almost directly with D since the BFN-noise is fairly constant (TBFN = 300 - 320 K), whereas the repeater noise TRP remains negligable due to the implemented BFN-gain.

REFERENCES [l] A. J. Simmons, D. G. Bodnar, Gain of Active Antenna Systems :

Antenna Standards Comittee Requests Inputs, IEEE APS-Newsletter, Oct. 1989, p. 62 J. J. Lee, G/T and Noise Figure of Active Array Antennas, IEEE Transactions, Vol. AP-41, No. 2, 1993, pp. 241-244 F. T. Ulaby, R. K. Moore, A. K. Fung, Microwave Remote Sensing, Addison-Wesley Publishing Comp., 1981, Vol. I , pp. 241-244

[2]

[3]

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